Magnet roller

ABSTRACT

To provide a magnet roller having a high magnetic force at low cost even if magnetized in a relatively weak magnetic field, having a magnetic force (800 G or higher) sufficient for development even if a plurality of magnetic poles are provided in a main magnetic pole part, and having good resistance to oxidation. The magnet roller ( 1 ) of the present invention includes a body part ( 3 ) and a shaft part ( 2 ) supporting both ends of ( 3 ). A plurality of magnetic poles are provided in a magnetized manner in an outer peripheral face of ( 3 ), made up of a rare-earth bond magnet made of rare-earth magnetic powder having a composite phase of a hard magnetic phase and a soft magnetic phase both magnetically exchange-interacting with each other and having a coercive force (iHc) of 5 KOe or lower and a residual magnetic flux density (Br) of 5 KG or higher.

RELATED APPLICATIONS

This application is a nationalization of PCT application PCT/JP00/05939filed Sep. 1, 2000. This application claims priority from the PCTapplication and Japan Application Serial No. H12(2000)-000960 filed Jan.6, 2000.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a magnet roller incorporated in anelectrophotographic device using an electrophotographic process in animage forming apparatus such as a copying machine, laser printer, orfacsimile receiving equipment or the like.

2. Description of the Related Art

A magnet roller incorporated in an electrophotographic device is used asa development roller which performs development by supplying toner to anelectrostatic latent image carrier and visualizing an electrostaticlatent image or a cleaning roller which removes residual toner on theelectrostatic latent image carrier after the visualized toner image istransferred to a sheet of paper. For example, when a magnet roller isused as a development roller, as shown in FIG. 9, a magnet roller 31 isconstructed so that a body part 33 thereof made of a magnet material isformed around a shaft part 32, and is incorporated in a hollowcylindrical sleeve 34 made of an aluminum alloy etc. In the outerperipheral face of the body part 33 of the magnet roller 31 are provideda plurality of magnetic poles in a magnetized manner. A magnetic polehaving a highest surface magnetic flux density of these magnetic polesis called a main magnetic pole, and is often used as a development pole.Conventionally, there has mainly been used a magnet roller in which amain magnetic pole (development pole) consisting of one magnetic pole isprovided in the body part, and the surface magnetic flux density curve(magnetic force distribution curve) thereof exhibits a single high peak,or a magnet roller in which two magnetic poles with the same polarityare provided as main magnetic poles (development poles), and themagnetic force distribution curve thereof exhibits two high peaks (Wpeak).

The body part of the conventional magnet roller is made up of a bondmagnet that is formed by injection molding or extrusion molding amaterial produced by mixing a resin binder such as a thermoplastic resinwith strontium-based or barium-based ferrite magnetic powder orrare-earth magnetic powder (typical examples are Nd—Fe—B based magneticpowder and Sm—Co based magnetic powder). The magnetic characteristicsnecessary for the magnet roller are provided by applying an externalmagnetic field to the body part of the magnet roller to magnetize it atthe time of molding or after the molding.

However, the conventional magnet roller mainly has problems of thefollowing items (1) to (4).

(1) The magnet roller using ferrite-based magnetic powder cannot meetthe requirement for high magnetic force. For the magnet roller in whichthe main magnetic pole is formed by a single magnetic pole, the magneticforce of the magnet roller having, for example, an outside diameter of13.6 mm is 850 G at the maximum, and, even if a strong magnetic field(about 30 KOe) is applied at the time of magnetization, it is difficultto obtain a high magnetic force of 850 G or higher because of magneticsaturation.

(2) In recent years, a magnet roller in which the main magnetic pole isformed by a plurality of magnetic poles has been developed. However, themagnetic force at the main magnetic pole thereof is 600 G or lower, andthus the magnetic pole cannot have a high magnetic force capable ofsufficiently playing a role as a development pole. The reason forforming the main magnetic pole by a plurality of magnetic poles is thatthe range of chain phenomenon of developer in the circumferentialdirection is wide, which offers an advantage of increasing thedevelopment efficiency.

(3) On the other hand, the magnet roller using rare-earth magneticpowder is barely able to provide a low magnetic force (about 700 G) bythe magnetization in a weak magnetic field because the coercive force ofrare-earth magnetic powder is relatively high (intrinsic coercive force(iHc): 5 KOe or higher). Therefore, a strong magnetic field (about 20 to30 KOe) must be applied to obtain a high magnetic force, so that amagnetizing apparatus must inevitably be large in size and require highpower, which results in complicated magnetizing process and high cost.

(4) The conventional rare-earth magnetic powder has a low Curie point ofabout 330° C., so that the use limit temperature thereof is restrictedto a low temperature of about 130° C. Also, the conventional rare-earthmagnetic powder has poor corrosion resistance and oxidation resistance,so that rust etc. are formed to decrease the magnetic characteristics.In order to prevent rust etc. from being formed, a surface coating suchas plating is needed, which brings about an increased cost.

SUMMARY OF THE INVENTION

The present invention has been achieved to solve the above problems, andaccordingly an object thereof is to provide a magnet roller capable ofobtaining a high magnetic force even if the magnet roller is magnetizedin a relatively weak magnetic field and capable of being produced at alow cost. In particular, another object of the present invention is tomake the magnetic force of a main magnetic pole formed by a singlemagnetic pole a high magnetic force of 850 G or higher even if themagnet roller is magnetized in a weak magnetic field of 15 KOe or lowerand to make the magnetic force of a main magnetic pole formed by aplurality of poles at a practically sufficient level. At the same time,still another object of the present invention is to provide a magnetroller in which a surface coating such as plating is not needed, and thecorrosion resistance and oxidation resistance are high.

To attain the above objects, the inventor paid attention to a“nanocomposite magnet” made up of a soft magnetic material having a lowcoercive force and a hard magnetic material, in which the crystal grainsize of the soft magnetic material is on the order of nanometer, andcarried out studies earnestly on the magnet of this type. As a result,the inventor found that the nanocomposite magnet is suitable as a magnetmaterial for a magnet roller, and came up with the present invention.

Specifically, the present invention provides a magnet roller comprisinga body part and a shaft part supporting both ends of the body part, inwhich a plurality of magnetic poles are provided in a magnetized mannerin the outer peripheral face of the body part, wherein the whole or apart of the body part is made up of a rare-earth bond magnet made ofrare-earth magnetic powder, having a composite phase of a hard magneticphase and a soft magnetic phase both magnetically exchange-interactingwith each other and having a coercive force (iHc) of 5 KOe or lower anda residual magnetic flux density of 5 KG or higher, and a resin binder.Therefore, there can be obtained a magnet roller having magneticcharacteristics of a low coercive force (iHc) provided by the presenceof soft magnetic phase and a high residual magnetic flux density (Br)provided by magnetic exchange-interaction.

Also, the rare-earth magnetic powder preferably consists of exchangespring magnetic powder. “Exchange spring magnetism” is defined as amagnetic property that when a large amount of soft magnetic phase existsin a magnet, crystal grains of this soft magnetic phase and a hardmagnetic phase are connected to each other by magneticexchange-interaction, by which the magnetization of soft magnetic phase,which intrinsically has only a low coercive force and is easily reversedin a reverse magnetic field, becomes difficult to reverse even in thereverse magnetic field, and a mode looking as if both phases areconnected to each other by a spring and thus a single phase consistingof hard magnetic phase only is exhibited (for example, see R. Coehoorn,K. H. J. Buschow et al.: J. de Phys., 49 (1988) C8-669).

The rare-earth magnetic powder using rare-earth element—iron—boroncompound phase as the hard magnetic phase and iron phase or iron—boroncompound phase as the soft magnetic phase, or the rare-earth magneticpowder using rare-earth element—iron—nitrogen compound phase as the hardmagnetic phase and iron phase as the soft magnetic phase is suitable.Since the rare-earth magnetic powder of this kind contains a largeamount of soft magnetic phase, the Curie point, which is an index oftemperature dependence of residual magnetization, is mainly governed bythe temperature dependency of soft magnetic phase. Therefore, the Curiepoint of the rare-earth magnetic powder takes a high value of about 400°C. or higher, and the temperature dependency of residual magnetizationbecomes low, so that the use limit temperature can be made as high as200° C. or higher.

Also, it is preferable that 1 to 16 wt % of cobalt (Co) be added to therare-earth magnetic powder. Thereby, a bond magnet manufactured of therare-earth magnetic powder is caused to contain more Co than theconventional rare-earth Nd—Fe—B based magnet consisting mainly of a hardmagnetic phase, so that the corrosion resistance and oxidationresistance are increased, and also the occurrence of rust etc. can beprevented without a surface coating such as plating. Specifically, ifthe Co content is lower than 1 wt %, the oxidation resistance etc. ofthe bond magnet decrease so that rust etc. are liable to be formed. Onthe other hand, if the Co content exceeds 16 wt %, the coercive force ofthe bond magnet decreases, so that it is difficult to maintain themagnetic characteristics necessary for the magnet roller.

Also, when a main magnetic pole is formed by a plurality of magneticpoles, it is preferable that the polarities of the adjacent magneticpoles of the magnetic poles forming the main magnetic pole be madereverse to each other. By making the polarities of the adjacent magneticpoles reverse to each other, the reversion (rotation) of developercaused by a change of magnetic polarity in the development zone (a zonein which developer chains toward a photosensitive material on the mainmagnetic pole) is generated with a rotation of the magnet roller in use,which offers an advantage that the supply efficiency of developer to aphotosensitive material can be increased.

In order to control the magnetic force distribution of magnet roller, itis preferable that a magnet piece made up of the rare-earth bond magnetbe provided in a groove formed along the axis near the main magneticpole in the outer peripheral face of the body part. This rare-earth bondmagnet can be formed by a single or a plurality of magnet pieces. Whenthe main magnetic pole is formed by a plurality of rare-earth bondmagnet pieces, it is preferable that the polarities of the adjacentrare-earth bond magnet pieces be make reverse to each other.

Also, the magnet roller may be formed by bondedly providing a pluralityof magnet pieces each consisting of the rare-earth bond magnet on theouter peripheral face of the shaft part. For example, the magnet rollermay be formed by bonding magnet pieces having a C-shaped cross section,which consist of the conventional ferrite resin magnet or the like inthe outer peripheral face of the shaft part, and by fitting a single ora plurality of rare-earth bond magnet pieces in a C-shaped openingportion (near the main magnetic pole) of the magnet piece.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic sectional view showing one embodiment of a magnetroller in accordance with the present invention;

FIG. 2 is a schematic view showing another embodiment of the magnetroller in accordance with the present invention;

FIG. 3 is a schematic view showing still another embodiment of themagnet roller in accordance with the present invention;

FIG. 4 is a schematic view showing still another embodiment of themagnet roller in accordance with the present invention;

FIG. 5 is a schematic view showing still another embodiment of themagnet roller in accordance with the present invention:

FIG. 6 is a schematic view showing still another embodiment of themagnet roller in accordance with the present invention;

FIG. 7 is a schematic view showing magnetic force distribution around acircumference of a magnet roller of an example;

FIG. 8 is a schematic view showing magnetic force distribution around acircumference of a magnet roller of another example; and

FIG. 9 is a schematic sectional view of a conventional developmentroller incorporated in a sleeve.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

Embodiments of a magnet roller in accordance with the present inventionwill now be described with reference to the accompanying drawings.

Referring to the schematic sectional view of FIG. 1, a magnet roller 1in accordance with the present invention is constructed so that a bodypart 3 is formed around a shaft part 2 made of stainless steel, aluminumalloy, resin, or the like. The body part 3 is made up of a rare-earthbond magnet made of rare-earth magnetic powder having a composite phaseof a hard magnetic phase and a soft magnetic phase both magneticallyexchange-interacting with each other and having a coercive force (iHc)of 5 KOe or lower and a residual magnetic flux density (Br) of 5 KG orhigher. The rare-earth magnetic powder is preferably exchange springmagnetic powder in which the crystal grain size of soft magnetic phaseis regulated to several tens of nanometers in order to make theexchange-interaction effective. Also, a plurality of magnetic poles(four poles of N₁, S₂, N₂ and S₁) are provided in a magnetized manner inthe outer peripheral face of the body part 3. A magnetic pole having thehighest magnetic force, of these magnetic poles, is a main magnetic pole(N₁ pole in this embodiment). Although four magnetic poles are providedat equal intervals in this embodiment, the number of poles and theposition of pole are not subject to any restriction in the presentinvention, and the number of poles and the position of pole can be setappropriately according to the desired magnetic characteristics; forexample, five poles or six poles may be provided.

As a magnet material for the body part 3 of the magnet roller, a mixturein which 5 to 50 wt % of resin binder is mixed with 50 to 95 wt % ofrare-earth magnetic powder is used as a main material, and as necessary,a silane-based or titanate-based coupling agent serving as a surfacetreatment agent of magnetic powder, an amide-based lubricant forimproving the flowability of molten magnet material, a stabilizing agentfor preventing the thermal decomposition of resin binder, a flameretardant, or the like is preferably added. If the content of therare-earth magnetic powder is less than 50 wt %, the magneticcharacteristics of magnet roller is decreased by a shortage of magneticpowder, so that a desirable magnetic force (850 G or higher at the mainmagnetic pole) is not obtained. If the content thereof exceeds 95 wt %,binder falls short, so that the formability of the body part 3 isimpaired. Also, as the resin binder, one kind or two or more kinds ofethylene-ethyl acrylate resin, polyamide resin, polyethylene resin,polystyrene resin, PET (polyethylene terephthalate), PBT (polybutyleneterephthalate), PPS (polyphenylene sulfide), EVA (ethylene-vinyl acetatecopolymer), EVOH (ethylene-vinylalcohol copolymer), PVC (polyvinylchloride), or the like, or one kind-or two or more kinds of epoxy resin,phenolic resin, urea resin, melamine resin, furan resin, unsaturatedpolyester resin, polyimde resin, and other thermosetting resins can beused mixedly.

As the rare-earth magnetic powder, exchange spring magnetic powder ofrare earth element (R)—iron (Fe)—nitrogen (N) alloy or rare earthelement (R)—iron (Fe)—boron (B) alloy containing a hard magnetic phaseand a soft magnetic phase is preferably used. Also, exchange springmagnetic powder of rare earth element (R)—iron (Fe)—cobalt (Co) alloymay be used. As the aforementioned R, Sm or Nd is preferably used, andbesides, one kind or two or more kinds of Pr, Dy and Tb can be usedcombinedly. Also, in order to enhance the magnetic characteristics byreplacing some of the aforementioned Fe, one or two or more kinds ofelements of Co, Ni, Cu, Zn, Ga, Ge, Al, Si, Sc, Ti, V, Cr, Mn, Zr, Nb,Mo, Tc, Ru, Rh, Pd, Ag, Cd, In, Sn, Sb, Hf, Ta, W, Re, Os, Ir, Pt, Au,Hg, Tl, Pb, Bi, etc. may be added. More specifically, exchange springmagnetic powder such as Nd—Fe—B based alloy (soft magnetic phase: Fe—Balloy, αFe), Sm—Fe—N based alloy (soft magnetic phase: αFe), AndNd—Fe—Co—Cu—Nb—B based alloy (soft magnetic phase: Fe—B alloy, αFeetc.), and Nd—Fe—Co based alloy (soft magnetic phase: αFe etc.) issuitable. In particular, from the viewpoint of decreasing coercive force(iHc) and increasing residual magnetic flux density (Br), exchangespring magnetic powder of Nd₄Fe₈₀B₂₀ alloy (soft magnetic phase: Fe₃B,αFe) or Sm₂Fe₁₇N₃ alloy (soft magnetic phase: αFe) is preferable.

Also, it is preferable that especially 1 to 16 wt % Co and further 3 to13 wt % Co be added to the aforementioned exchange spring magneticpowder. For a bond magnet manufactured by the rare-earth magneticpowder, the magnetic characteristics are improved and the corrosionresistance and oxidation resistance are increased by this addition ofCo, so that the formation of rust etc. can be restrained without asurface coating on the outside surface thereof.

In order to manufacture the aforementioned exchange spring magneticpowder, a rapid quenching method, mechanical alloying method, or thelike is used. Specifically, there are cited a method in which eachfeedstock element is weighed, alloy powder obtained by mechanicalalloying is heat-treated, and as necessary, nitriding treatment isperformed, a method in which each feedstock element is weighed, an alloycontaining an amorphous or near-amorphous microstructure obtained byrapid quenching using the single roll method is ground and thensubjected to heat treatment to deposit crystal, and as necessary,nitriding treatment is performed, or the like methods. By properlycontrolling the quenching conditions (rolling speed, etc.), grindingconditions, heat treatment conditions (treatment time, temperature), orthe like conditions, exchange spring magnetic powder having a softmagnetic phase with a crystal grain size of several tens of nanometerscan be manufactured. The aforementioned nitriding treatment is necessarywhen R—Fe—N based exchange spring magnetic powder is manufactured.

The magnet roller in accordance with the present invention and a magnetpiece described later are formed by the extrusion molding method or theinjection molding method using pellets obtained by melting and kneadingthe aforementioned magnet material. Alternatively, they may be formed bythe compression molding method using the aforementioned magnet material.The magnetization of the magnet roller and the magnet piece is providedby performing oriented magnetization simultaneously with the injectionmolding or extrusion molding, by performing magnetization again afterdemagnetization performed once to remove internal strain or tofacilitate mold release, or by performing magnetization after moldingwithout oriented magnetization at the time of molding. In forming andassembling the magnet roller, the magnet roller can be formed byintegrally molding the body part and the shaft part thereof, byinstalling the shaft part to both ends of the body part or penetratinglyinstalling the shaft part in the cylindrically-shaped body part, or bybonding a magnet piece formed into an irregular shape such assemicylindrical shape or fan shape to the shaft part with a circular,elliptical, or polygonal cross-sectional shape.

It is preferable that, for example, as shown in the schematic sectionalview of FIG. 2 (or FIG. 3), a magnet piece 7 (11) formed of theaforementioned rare-earth bond magnet be provided in a groove 6 (10)formed along the axis near the main magnetic pole in a body part 5 (9)of a magnet roller. This magnet piece preferably is a square piece 7having a square shaped cross-section as shown in FIG. 2 or a semi-fanshaped piece 11 having a semi-fan shaped cross section as shown in FIG.3. The piece having such a shape is easily made common to a piece foranother magnet roller, and also has high formability and bondability.

When the main magnetic pole is composed of a plurality of magneticpoles, as shown in the schematic sectional view of FIG. 4, the mainmagnetic poles can be formed by forming a body part 13 provided with agroove 14 having a fan-shaped cross section, which is provided with aplurality of magnetic poles (N pole, S pole, N pole, S pole) in thesurface thereof, on the outer peripheral face of a shaft part 12, and bybondedly providing magnet pieces 15A, 15B and 15C consisting of theaforementioned rare-earth bond magnet and each having S pole, N pole,and S pole in the surface thereof in the groove 14 so that the magneticpolarities of the adjacent magnet pieces are reverse to each other.Alternatively, as shown in the schematic sectional view of FIG. 5, themain magnetic poles can be formed by bonding a ferrite bond magnet 17having a C-shaped cross section, which is provided in a magnetizedmanner with a plurality of magnetic poles (N pole, S pole, N pole, Spole) in the surface thereof, to the outer peripheral face of a shaftpart 16, and by bondedly providing magnet pieces 18A, 18B and 18Cconsisting of the aforementioned rare-earth bond magnet and each havingS pole, N pole, and S pole in the surface thereof in a C-shaped openingportion (near the main magnetic pole) of the ferrite bond magnet 17 sothat the magnetic polarities of the adjacent magnet pieces are reverseto each other.

Also, as shown in the schematic sectional view of FIG. 6, a magnetroller can be manufactured by bondedly providing magnet pieces 21A, 21Band 21C consisting of the aforementioned rare-earth bond magnet and eachhaving S pole, N pole, and S pole in the surface thereof on the outerperipheral face of a shaft part 19 to form the main magnetic poles, andby bonding ferrite bond magnet pieces 20A, 20B, 20A′ and 20B′ havingother magnetic poles to the outer peripheral face of the shaft part 19.

In the present invention, the shape of magnet piece is not subject toany restriction, and the shape thereof can be changed appropriatelyaccording to the desired magnetic characteristics (magnetic force,magnetic force distribution waveform, etc.). Also, in order tomanufacture the above-described magnet roller, what we call thetwo-color molding method can be used in which two injection moldingmachines are used, and after the body part is molded by a firstinjection molding machine, the magnet piece is molded in the groove by asecond injection molding machine. This method is effective insignificantly simplifying the manufacturing process.

As the magnetic field orientation of the magnet piece, randomorientation, straight orientation as indicated by the arrows in FIGS. 2Band 3B, and radial orientation as indicated by the arrows in FIGS. 2Cand 3C are typical examples. Also, not shown in the drawings, themagnetic force distribution of magnet roller can be controlled byconverging the magnetic flux density of applied magnetic field tocontrol the oriented magnetization amount at any place on both sidefaces and the back side face to the surface side face of the magnetpiece 7 (11). This control is effective, for example, when the magneticforce distribution waveform in the main magnetic pole is madeasymmetric. Also, the magnet roller can be formed by combining magnetpieces each having a different orientation, and the combination may beselected appropriately according to the required specification.

The use of isotropic magnetic powder as the rare-earth magnetic powderin accordance with the present invention and the magnetization of themagnet roller performed after molding are desirable because a desiredposition of the magnet roller is easily magnetized into a desiredmagnetic force. In particular, since a magnetic circuit need not beformed in the molding apparatus, the molding die is low in cost. Also,since there is no deformation of bond magnet caused by the appliedmagnetic field at the time of molding, the dimensional accuracy aftermolding is high, so that magnetization is easy to perform, and the poleposition can be set with high accuracy.

EXAMPLES

The following is a description of more specific examples in accordancewith the present invention and comparative examples. The followingexamples do not impose any restriction on the present invention.

The magnet roller of examples 1 to 3 and comparative examples 1 to 4described in detail below was manufactured by mixing, melting, andkneading 10 wt % of resin binder (nylon 12) and 90 wt % of magneticpowder to form pellets, by forming a roller (diameter: 13.6 mm, overalllength: 320 mm) by injection molding using these pellets, and then byapplying an external magnetic field to magnetize four poles (N₁, S₂, N₂,S₁) as shown in FIG. 7. The magnetic force distribution of this magnetroller was measured in a state of being incorporated in a sleeve 22 madeof an aluminum alloy. In FIG. 7, the N₁ pole is the main magnetic pole,and reference numeral 23 denotes a shaft part, 24 denotes a body part,and 25 denotes a distribution waveform of magnetic force. Point Adesignates the highest value of the magnetic force distribution at themain magnetic pole.

Example 1

A magnet roller was manufactured by mixing and kneading nylon 12 used asa resin binder and exchange spring magnetic powder of Nd₄Fe₈₀B₂₀(intrinsic coercive force iHc: 3.0 KOe, residual magnetic flux densityBr: 12 KG, Co content: 2 wt %) used as magnetic powder to form pellets,by forming a roller by injection molding, and then by magnetizing theroller with an intensity of applied magnetic field of 8 to 15 KOe.

Example 2

A magnet roller was manufactured by the same way as that of example 1except that exchange spring magnetic powder ofNd₅Fe₇₁Co₅Cu_(0.5)Nb₁B_(17.5) (intrinsic coercive force iHc: 4.8 KOe,residual magnetic flux density Br: 5.2 KG, Co content: 6 wt %) was usedas magnetic powder.

Example 3

A magnet roller was manufactured by the same way as that of example 1except that exchange spring magnetic powder of Sm₂Fe₁₇N₃ (coercive forceiHc: 4.0 KOe, residual magnetic flux density Br: 7.8 KG, Co content: 1wt %) was used as magnetic powder.

Comparative Example 1

A magnet roller was manufactured by the same way as that of example 1except that ferrite magnetic powder of SrO·6Fe₂O₃ (coercive force iHc: 3KOe, residual magnetic flux density Br: 4.8 KG, Co content: 0 wt %) wasused as magnetic powder.

Comparative Example 2

A magnet roller was manufactured by the same way as that of example 1except that ferrite magnetic powder of SrO·6Fe₂O₃ (coercive force iHc: 3KOe, residual magnetic flux density Br: 4.8 KG, Co content: 0 wt %) wasused as magnetic powder, and the intensity of applied magnetic field was20 to 30 KOe.

Comparative Example 3

A magnet roller was manufactured by the same way as that of example 1except that rare-earth magnetic powder (Nd_(13.5)Fe_(1.7)B_(4.8);coercive force iHc: 14 KOe, residual magnetic flux density Br: 8.4 KG,Co content: 0.5 wt %) was used as magnetic powder.

Comparative Example 4

A magnet roller was manufactured by the same way as that of example 1except that rare-earth magnetic powder (Nd_(13.5)Fe_(1.7)B_(4.8);coercive force iHc: 14 KOe, residual magnetic flux density Br: 8.4 KG,Co content: 0.5 wt %) was used as magnetic powder, and the intensity ofapplied magnetic field was 20 to 30 KOe.

Next, a magnet roller of example 4 and comparative example 5 describedin detail below is formed by bonding magnet pieces 29A to 29C, 28A, 28B,28A′ and 28B′ to each other on the outer peripheral face of a shaft part26 as shown in FIG. 8 (diameter: 13.6 mm, overall length: 320 mm). Eachof the magnet pieces was manufactured by mixing, melting, and kneading10 wt % of resin binder (nylon 12) and 90 wt % of magnetic powder toform pellets, by molding the magnet piece by injection molding usingthese pellets, and then by applying an external magnetic field formagnetization after the molding. In FIG. 8, the S₁ pole, N₁ pole, and S₂pole are main magnetic poles, and reference character 26 denotes a shaftpart, 28A, 28B, 28A′ and 28B′ denote ferrite bond magnet pieces, 29A to29C denote rare-earth bond magnet pieces, and 30 denotes a magneticforce distribution waveform. Points B, C and D designate the highestvalue of magnetic force distribution at the main magnetic poles.

In FIG. 8, the pole-to-pole angle (θ₂) between the magnetic force peakposition (point B) at the pole on the upstream side of developertransfer and the magnetic force peak position (point D) at the pole onthe downstream side of developer transfer was set at 60 degrees.Incidentally, the pole-to-pole angle (θ₁) between point B and point C is30 degrees. If the pole-to-pole angle (θ₂) between point B and point Dexceeds 60 degrees, the chain of developer near point C which faces aphotosensitive material is coarse, which is the same as the state ofchain in the development zone of the conventional magnet roller. At thesame time, the reversion (rotation) of developer caused by a change ofmagnetic polarity is inactive. Therefore, the amount of toner suppliedto the photosensitive material decreases, so that high picture qualitycannot be achieved. On the other hand, if the pole-to-pole angle (θ₂) issmaller than 30 degrees, a high magnetic force cannot be obtained evenif the aforementioned rare-earth bond magnet pieces are used for themain magnetic pole part, and thus it was verified that it is difficultto achieve high picture quality.

Example 4

As magnetic powder for magnet pieces having the S₁ pole, N₁ pole, and S₂pole, forming the main magnetic poles, exchange spring magnetic powderof Nd₄Fe₈₀B₂₀ (intrinsic coercive force iHc: 3.0 KOe, residual magneticflux density Br: 12 KG, Co content: 2 wt %) was used, and as a resinbinder, nylon 12 was used. Both of the materials were mixed and kneadedto form pellets, and magnet pieces having a fan-shaped cross sectionwere formed by injection molding using these pellets. After the magnetpieces were magnetized by applying an external magnetic field, themagnet pieces were bonded to each other on the outer peripheral face ofthe shaft part 26.

Also, as magnetic powder for magnet pieces having the N₂ pole, S₃ pole,S₄ pole, and N₃ pole, forming magnetic poles other than the mainmagnetic poles, ferrite magnetic powder of SrO·6Fe₂O₃ (coercive forceiHc: 3 KOe, residual magnetic flux density Br: 4.8 KG) was used, and asa resin binder, nylon 12 was used. Both of the materials were mixed andkneaded to form pellets, and magnet pieces having a fan-shaped crosssection were formed by injection molding. After oriented magnetizationwas effected simultaneously with the molding, the magnet pieces werebonded to each other on the outer peripheral face of the shaft part 26.

Comparative Example 5

The exchange spring magnetic powder used for the magnet pieces formingthe main magnetic poles (S₁ pole, N₁ pole, and S₂ pole) in theabove-described example 4 was changed to ferrite magnetic powder ofSrO·6Fe₂O₃ (coercive force iHc: 3 KOe, residual magnetic flux densityBr: 4.8 KG, Co content: 0 wt %). This ferrite magnetic powder and nylon12 were mixed and kneaded to form pellets, and magnet pieces having afan-shaped cross section were formed by injection molding using thesepellets. Thereafter, a magnet roller was manufactured in the same way asthat of example 4 except that oriented magnetization was effectedsimultaneously with the molding.

The magnetic force distributions of the above-described examples andcomparative examples were measured by using a gauss meter, by arrangingprobes in positions 1.2 mm distant from the surface of magnet roller inthe radial direction (positions 8.0 mm distant from the center axis ofmagnet roller in the radial direction), and by turning the magnet rollerin the circumferential direction. The magnetic characteristics of themagnetic powder and magnet rollers used in examples and comparativeexamples are given in Tables 1 and 2, and the oxidation resistance wasgiven in Table 3. Regarding the oxidation resistance, after themanufactured magnet roller was left in air for 168 hours, the presenceof rust on the surface of magnet roller was checked visually. Themagnetic characteristics given in Tables 1 and 2 include “intrinsiccoercive force iHc” and “residual magnetic flux density Br” of magneticpowder, “intrinsic coercive force iHc after molding” and “residualmagnetic flux density Br” of magnet roller, “intensity of magnetic fieldfor magnetization of main magnetic pole”, and “magnetic force of mainmagnetic pole” at points A to D after magnetization.

TABLE 1 Magnetic charac- Magnetic teristics characteristics of magneticafter powder molding Re- Re- sidual sidual Mag- mag- mag- Intensity ofnetic netic netic magnetic field force of Coer- flux Coer- flux for maincive den- cive den- magnetization mag- force sity force sity of mainnetic iHc Br iHc Br magnetic pole pole (G) (KOe) (KG) (KOe) (KG) (KOe)Point A Example 1  3.0 12.0  3.0 7.0 15 1800 Example 2  4.8  5.2  4.83.2 15  850 Example 3  4.0  7.8  4.0 4.6 15 1200 Comparative  3.0  4.8 3.0 3.0 15  800 example 1 Comparative  3.0  4.8  3.0 3.0 30  800example 2 Comparative 14.0  8.4 14.0 5.1 15  700 example 3 Comparative14.0  8.4 14.0 5.1 30 1350 example 4

TABLE 2 Magnetic Magnetic characteristics of characteristics afterIntensity of magnetic powder molding magnetic field Residual Residualfor magnetic magnetic magnetization Coercive flux Coercive flux of mainMagnetic force of main force density force density magnetic polemagnetic pole (G) iHc (KOe) Br (KG) iHc (KOe) Br (KG) (KOe) Point BPoint C Point D Example 4 3.0 12.0 3.0 7.0 15.0 800 950 800 Comparative3.0  4.8 3.0 3.0 15.0 450 550 450 example 5

TABLE 3 Example 1 Example 2 Example 3 Example 4 Presence Absent AbsentAbsent Absent or absence of rust Com- Com- Com- Com- Com- parativeparative parative parative parative example 1 example 2 example 3example 4 example 5 Presence Absent Absent Present Present Absent orabsence of rust

Evaluation of Examples 1 to 3 and Comparative Examples 1 to 4 in Whichthe Main Magnetic Pole is Formed of a Single Pole

As is apparent-from the results given in Table 1, for the magnet rollerusing exchange spring magnetic powder in examples 1 to 3, whenmagnetization was effected in a weak magnetic field (15 KOe), a magneticforce of 850 G or higher was obtained in all of the examples; on theother hand, for the magnet roller made up of a conventional ferrite bondmagnet in comparative examples 1 and 2, a magnetic force of only 800 Gwas obtained even if magnetization was effected in a weak magnetic field(15 KOe) or a strong magnetic field (30 KOe). Also, for the magnetroller made up of a conventional rare-earth bond magnet in comparativeexamples 3 and 4, when magnetization was effected in a strong magneticfield (30 KOe) as in comparative example 4, a magnetic force as high as1350 G was obtained; however, when magnetization was effected in a weakmagnetic field (15 KOe) as in comparative example 3, a magnetic force ofonly 700 G was obtained. Therefore, it was verified that the magnetrollers of these examples can provide a high magnetic force (850 G orhigher) by means of magnetization effected in a weak magnetic field (15KOe or lower).

Also, as is apparent from the results given in Table 3, in examples 1 to3 in which exchange spring magnetic powder was used and in comparativeexamples 1 and 2 in which conventional ferrite magnetic powder was used,rust was not formed at all; on the other hand, in comparative examples 3and 4 in which rare-earth magnetic powder mainly consisting of a hardmagnetic phase, the occurrence of rust was confirmed.

Evaluation of Example 4 and Comparative Example 5 in Which the MainMagnetic Pole is Formed of Three Poles

As is apparent from the results given in Table 2, in example 4, themagnetic forces of all three poles constituting the main magnetic polewere 800 G or higher; on the other hand, in comparative example 5, themagnetic forces of all three poles constituting the main magnetic polewere only 600 G or lower. Therefore, it was verified that for the magnetroller of example 4, even if the three magnetic poles are arranged inthe range of 60 degrees near the main magnetic pole, a high magneticforce (800 G or higher) enough for development can be obtained, and thechain of developer near point C is dense.

Also, according to Table 3, in example 4 in which exchange springmagnetic powder was used and in comparative example 5 in whichconventional ferrite magnetic powder was used, rust was not formed atall.

According to the magnet roller in accordance with the present invention,since the body part thereof is made up of a rare-earth bond magnet madeof rare-earth magnetic powder having a composite phase of a hardmagnetic phase and a soft magnetic phase both magneticallyexchange-interacting with each other and having a coercive force (iHc)of 5 KOe or lower and a residual magnetic flux density of 5 KG orhigher, the low coercive force of soft magnetic phase and magnetizationgenerally higher than the hard magnetic phase can be utilized, so that ahigh magnetic force can be obtained even if magnetization is effected ina weak magnetic field. Especially when the main magnetic pole is formedby a single pole, even if magnetization is effected in a weak magneticfield of 8 to 15 KOe, a high magnetic force of 850 G or higher can beobtained. Also, even when the main magnetic pole is formed by aplurality of poles, a high magnetic force of 800 G or higher can beobtained. Therefore, a magnet roller having high development efficiencycan be provided. Since the main magnetic pole can be magnetized in aweak magnetic field, the large size and high power of magnetizingapparatus can be avoided, so that a magnet roller having excellentmagnetic characteristics can be provided while the manufacturing cost iskept low.

Also, by adding 1 to 16 wt % of cobalt to the rare-earth magneticpowder, a magnet roller that does not require a surface coating such asplating and has high corrosion resistance and oxidation resistance canbe obtained, so that the magnet roller can have stable magneticcharacteristics for a long period of time.

Further, by forming the main magnetic pole by a plurality of magneticpoles and making the polarities of adjacent magnetic poles of themagnetic poles forming the main magnetic pole reverse to each other, thechain of developer near the main magnetic pole can be made dense at thetime of use, and also the reversion (rotation) of developer caused by achange of magnetic polarity in the development zone is active, so thatthe supply efficiency of developer to a photosensitive materialincreases, which can achieve high picture quality.

What is claimed is:
 1. A magnet roller comprising: a body part; and ashaft part supporting both ends of said body part, in which a pluralityof magnetic poles are provided in a magnetized manner in the outerperipheral face of said body part, wherein the whole or a part of saidbody part is made up of a rare-earth bond magnet made of rare-earthmagnetic powder, having a composite phase of a hard magnetic phase and asoft magnetic phase both magnetically exchange-interacting with eachother and having a coercive force (iHc) of 5 KOe or lower and a residualmagnetic flux density of 5 KG or higher, and a resin binder.
 2. Themagnet roller according to claim 1, wherein said rare-earth magneticpowder consists of exchange spring magnetic powder.
 3. The magnet rolleraccording to claim 1 or 2, wherein rare-earth element—iron—boroncompound phase is used as said hard magnetic phase, and iron phase oriron—boron compound phase is used as said soft magnetic phase.
 4. Themagnet roller according to claim 1 or 2, wherein rare-earthelement—iron—nitrogen compound phase is used as said hard magneticphase, and iron phase is used as said soft magnetic phase.
 5. The magnetroller according to any one of claims 1 or 2, wherein 1 to 16 wt % ofcobalt is added to said rare-earth magnetic powder.
 6. The magnet rolleraccording to any one of claims 1 or 2, wherein a main magnetic pole isformed by a plurality of magnetic poles, and the polarities of theadjacent magnetic poles of said magnetic poles forming said mainmagnetic pole are made reverse to each other.
 7. The magnet rolleraccording to any one of claims 1 or 2, wherein a magnet piece made up ofsaid rare-earth bond magnet is provided in a groove formed along theaxis near the main magnetic pole in the outer peripheral face of saidbody part.
 8. The magnet roller according to any one of claims 1 or 2,wherein a plurality of magnet pieces each made up of said rare-earthbond magnet are bondedly provided on the outer peripheral face of saidshaft part.